Clinch joints for fastening sheets of deformable material such as sheet metal or the like have been used for over one hundred years. Various lanced clinch sheet metal joints are used today which are very similar in design as shown in U.S. Pat. No. 56,494--Gordon, issued in 1866. Over the years, clinching has experienced limited commercial utilization. Currently the most popular method of fastening overlapping segments of sheet metal together is spot welding. Spot welding poses problems, particularly in certain applications such as when using dissimilar metals, metals having protective coatings, metals with different thicknesses or hard to weld materials. Clinch joints are not problem free either and whether to use a spot weld or a clinch joint must be determined on an application by application basis. Clinch joints of the lanced variety shear the metal exposing the central portion of the sheet to the atmosphere. A lanced joint is not suitable when a leak proof joint is necessary, or where the metal sheets have corrosion resistant coatings or the joint must be pressure-tight, such as a beverage can tab top attachment.
Leak proof clinch joints are well known, but have yet to experience wide spread commercial uses. Leak proof clinch joints of a conventional design are shown in U.S. Pat. Nos. 3,359,935 Rosbottom; 3,579,809 Wolf; 4,459,735, Sawdon, and described in detail in SAE Technical Paper, A New Mechanical Joining Technique for Steel Compared to Spot J. M. Sawhill and S. E. Sawdon, No. 830128. Conventional leak proof clinch joints are formed by simultaneously drawing a pair of cup-shaped cavities nested one within the other between a punch and die. Once the cavities are drawn, the spot forming the central region of the cavity is squeezed axially to radially deform and interlock the nested cavities.
When forming clinch joints of either the lanced or leak proof type, it is important to accurately control the manufacturing and tolerances of the punch and die assembly used and to carefully maintain alignment. Many die assemblies are formed of a series of die segments which are outwardly radially biasable as the central spot of the clinch joint is axially compressed between a punch and an anvil. During the drawing and squeezing of the spot operation, very significant axial load is exerted on the die segments. When the die segments are massive, such as the hinge die segments utilized in Sawdon, axial load is not a problem. However, when a flexible die assembly is used, such as the one piece die button, as shown in U.S. Pat. No. 4,208,776, Schleicher or 4,569,111, Mutou when fabricated by assembly of the type illustrated in U.S. Pat. No. 4,614,017, Eckold, die segments break periodically if the segments are made very flexible and tend to hinder spot expansion if the die segments are stiff.
In order to minimize the complexity of the punch and die assembly, there have been several attempts to form clinch joints using a solid punch in combination with a solid non-radially expandable die set. One such example is U.S. Pat. No. 4,394,794, Shirey, which utilizes a dovetail shaped slot in the die in which the clinch joint once formed must be removed from the die transversely. Another example of a solid punch and die lance-type joint and forming apparatus is shown in Italian patent 526042, Rosca, which lances and subsequently deforms a joint central spot between the punch and die.
A leak proof joint using a solid punch and die is illustrated in U.S. Pat. No. 3,579,809, Wolf. Joints of this type tend to be good in shear strength or alternatively good in peel strength, but not strong in both due to the design compromises inherent with making a symmetrical joint.